The role of Atlantic ocean on the decadal- multidecadal variability of Asian summer monsoon Observational and paleoclimate evidences Observational and paleoclimate evidences Model experiments Model experiments Physical mechanisms Physical mechanisms Summary Summary Buwen Dong (NCAS-Climate, University of Reading) and Riyu Lu (IAP/Chinese Academy of Science)

Low frequency fluctuation of Asian summer monsoon: Weakened EASM in Rain Gauge Data. From Zhou (2007) JJA trend in unit of cm/50a from Northern drought and southern flood pattern Normalized time series of JJA precipitation averaged over the Yangtze River and Yellow River

Low frequency fluctuation of Asian summer monsoon: From Kripalani and Kulkarni (2001) No obvious long term trends in precipitation over three regions. Bear in mind the accuracy of precipitation in early part of the 20 th century.

What has caused northern drought and southern flood in China? Internal low frequency climate variability a) Upper tropospheric cooling in spring (the role of low frequency change in the NAO/AO and the stratosphere) b) Low frequency fluctuation of SST in Pacific (PDO or IPO) c) Low frequency fluctuation of snow cover/mass in Eurasia d) Anomalous SSTs associated with Atlantic Multidecadal Oscillation (AMO). External forcing a) Greenhouse gases b) Aerosols c) Land use change

The Atlantic Thermohaline Circulation (THC) A key Element of the Global Ocean Circulation Transports ~1.2PW heat from low to high latitude and helps to keep western Europe mild in winter and plays an important role in maintaining the mean climate.

Atlantic Multidecadal Oscillation (AMO) The AMO is characterized by opposite signs of SST anomalies in the North and South Atlantic and is thought to be related in part to multidecadal fluctuations of the Atlantic Thermohaline Circulation (THC). Sutton et al. (2005)

Observation evidences (JJA) Correspondence between AMO and AIR. Positive AMO phase with strong AIR and negative AMO phase with weak AIR. Both AIR and rainfall based on CRU data shows similar relationship to AMO. (mean=7-9.0mm day -1, sdv= mm day -1 )

Observation evidences (JJA) Correspondence between AMO, rainfall over East Asia, and EASM index on the multi- decadal time scale. Positive AMO phase are associated with above rainfall and positive EASM index and negative AMO phase with below rainfall and negative EASM index. Strong correspondence in the last years. (mean= mm day -1, sdv= mm day -1 )

Observation evidences (JJA) Relative to the AMO negative phase, summer surface air temperature over east Asia is warmer and precipitation is stronger during the positive phase of AMO. JJA air temperature JJA precipitation Positive AMO phase: and Negative AMO phase: Lu et al. (2006)

Paleoclimate evidence of the connection between Atlantic ocean and east Asian summer monsoon From Wang et al. 2004

The above observational evidences and paleoclimate records suggest a possible link between the North Atlantic ocean state and Asian summer monsoon intensity on multidecadal and millennial time scales. When the North Atlantic was cold, the main remote features are: (1) Wet conditions in tropical northeastern Brazil (1) Wet conditions in tropical northeastern Brazil (2) Weak Indian summer monsoon (2) Weak Indian summer monsoon (3) Weak east Asian summer monsoon (3) Weak east Asian summer monsoon Now we use a coupled atmosphere-ocean GCM with specially designed experiments to investigate this possible link and to try to elucidate possible mechanisms behand this link. Now we use a coupled atmosphere-ocean GCM with specially designed experiments to investigate this possible link and to try to elucidate possible mechanisms behand this link.

Regional Coupled Model Experiments 2x150 year experiments with HadCM3 coupled model (Atm: 2.5 o X3.75 o with 19 levels. Ocean: 1.25 o X1.25 o with 20 levels) 2x150 year experiments with HadCM3 coupled model (Atm: 2.5 o X3.75 o with 19 levels. Ocean: 1.25 o X1.25 o with 20 levels) Initial state from a 1700y control simulation Initial state from a 1700y control simulation +/- AMO: Atlantic SSTs relaxed to seasonally varying climatology from the coupled model +/- AMO: Atlantic SSTs relaxed to seasonally varying climatology from the coupled model Relaxation timescale = 2.5 days Relaxation timescale = 2.5 days

Role of Atlantic ocean on Asian summer monsoon Role of Atlantic ocean on Asian summer monsoon The results from regional coupled experiments are qualitatively in agreement with observational and paleoclimate evidences. Warm North Atlantic, dry condition over northeast Brazil, wet conditions over South Asia (Lu et al 2006).

Role of Atlantic ocean on Asian summer monsoon Role of Atlantic ocean on Asian summer monsoon Consistent with Goswami et al. (2006), Warm North Atlantic leads to a late withdraw of Indian summer monsoon. From Lu et al. (2006).

HadCM3 “water hosing” experiments HadCM3 (Atm: 2.5 o X3.75 o with 19 levels. Ocean: 1.25 o X1.25 o with 20 levels) HadCM3 (Atm: 2.5 o X3.75 o with 19 levels. Ocean: 1.25 o X1.25 o with 20 levels) One “water hosing” experiment has been performed by applying an anomalous freshwater flux 1.0 Sv for 100 years uniformly to the North Atlantic ocean between 50 o N and 70 o N, then stopped, and the model integrated for another 100 years. One “water hosing” experiment has been performed by applying an anomalous freshwater flux 1.0 Sv for 100 years uniformly to the North Atlantic ocean between 50 o N and 70 o N, then stopped, and the model integrated for another 100 years. One parallel control experiment of 200 year One parallel control experiment of 200 year

THC index THC rapidly weakens as a result of anomalous fresh water flux over North Atlantic. THC decreases by 70% for the 1.0Sv experiment.

Mean climate anomalies in JJA season in response to weakened THC Remote responses: Cooling over Maritime continents and Eurasia Cooling over Maritime continents and Eurasia Weakened WPSH and associated southly flow Weakened WPSH and associated southly flow Weakened Somali jet Weakened Somali jet Reduced precipitation over East Asia and South Asia Reduced precipitation over East Asia and South Asia From Lu and Dong (2008)

Seasonal cycle of precipitation over India (10N-25 o N, 70 o E-90 o ) and (b) East Asia (20 o N-35 o N, 105 o E-120 o E) Seasonal cycle of precipitation over India (10 o N-25 o N, 70 o E-90 o ) and (b) East Asia (20 o N-35 o N, 105 o E-120 o E) Both Indian summer monsoon precipitation and east Asian summer precipitation decrease when the North Atlantic is cold. The relative large change around onset and withdraw indicates a shortening of monsoon season. From Lu and Dong (2008).

A schematic diagram illustrates main processes involved to link the North Atlantic and Asian summer monsoon

Summary 1) Weakening of the Atlantic THC due to anomalous external freshwater forcing in the North Atlantic leads to a weakened Asian summer monsoon. 2) The influence is predominantly through atmospheric teleconnection, which induces coupled feedbacks over the Indian ocean and the western tropical Pacific. The changes in Eurasian continent also play a role. 3) This study suggests a non-local mechanism for the low frequency Asian summer monsoon variability. 4) Atlantic influence could help to explain the interdecadal variability of the Asian summer monsoon during the twentieth century 5) It could also help to explain previously documented paleoclimate evidences which suggested a link between the Asian summer monsoon and the North Atlantic ocean on the sub-orbital scales.

Further Questions In terms of Atlantic influence on Asian summer monsoon, what is the relative role of Eurasian land surface feedbacks and the role of coupled feedbacks in the Maritime continent and the western tropical Pacific? In terms of Atlantic influence on Asian summer monsoon, what is the relative role of Eurasian land surface feedbacks and the role of coupled feedbacks in the Maritime continent and the western tropical Pacific? Does the multidecadal fluctuation of Atlantic ocean influence monsoon intraseasonal variability and extremes? Does the multidecadal fluctuation of Atlantic ocean influence monsoon intraseasonal variability and extremes? Most climate models predict a weakening of the Atlantic THC for the 21st century when forced by increasing concentrations of greenhouse gases. What is the role of the weakened THC on the Asian summer monsoon and uncertainty of projected monsoon change? Most climate models predict a weakening of the Atlantic THC for the 21st century when forced by increasing concentrations of greenhouse gases. What is the role of the weakened THC on the Asian summer monsoon and uncertainty of projected monsoon change?